This invention relates to a process for producing meltblown polyolefin fibres for mechanical filtration This invention also relates to the meltblown polyolefin fibres when produced by the process of the invention, and to filters containing the meltblown polyolefin fibres.
Filters are widely used for industrial and medical purposes. Thus, for example, the filters may be air filters. The air filters may be in the form of industrial respirators, automative air filters, machine air filters, air conditioning filters, and clean room filters. The filters may alternatively be oilophobic mechanical filters such for example as hydraulic filters. The filters may alternatively be liquid barrier membrane filters using fibres which are calendered to control pore size. The filters may still further be medical air filters or face masks for surgeons.
The performance standards for air filters in Europe are known as Eurovent standards, and the performance standards in the USA are known as Ashrae standards. The standards challenge a filter over a defined period of time with increasing levels of dust, and they measure filtration efficiency, pressure drop and dust loading. The performance achieved by a filter is graded according to a pre-determined set of criteria with the grading ranging from 1 at the lowest level (giving a particle filtering efficiency of not more than 65%) to 14 at the highest level (giving a particle filtering efficiency of not less than 99.9995%).
Filters that operate with a Burovent/Ashrae standard of 6 and above have commonly used fibres made from glass. The glass fibres have an effective diameter of less than 5 microns in size and they rely-on mechanical characteristics for their filtration efficiency. It is also known to use polyolefin fibres having an effective diameter of more than 5 microns in size. Such polyolefin fibres provide only limited mechanical filtration efficiency in their mechanical state, and for Eurovent/Ashrae standards of 6 and above they have to be electrostatically charged in order to maintain their filtration efficiency.
There are two well known types of filters which use the glass fibres or the polyolefin fibres in the Eurovent/Ashrae range of 6 and above. The first of these filters is a bag filter which employs a surface area commonly less than 10 m.sup.2. These bag filters have traditionally used glass fibres as the filtration media but health concerns over recent years have led to a change to polyolefin fibres which are used in heavier weights than the glass fibres. The second type of filter is a pleated panel fitter which employs a surface area commonly between 8-20 m.sup.2. These pleated panel filters continue to be generally manufactured from glass fibres because the higher efficiencies of glass fibres for a lower weight enable the filters to be pleated, thereby incorporating a larger surface area for a given panel size.
The polyolefin fibres are usually meltblown polyolefin fibres. Irrespective of the type of, filters employed, two factors are involved, namely filtration efficiency and pressure drop. The filtration efficiency may be regarded as the number of filtered particles arrested by the fibres, whilst the pressure drop may be regarded as the load required to suck air through the filter. The filtration efficiency and the pressure drop can be controlled by machine settings during the production of the meltblown polyolefin fibres.
The known meltblown polyolefin fibres are usually produced by melting polyolefin granules in an extruder or meltbox, and then forcing the melted granules through a die under pressure. High pressure hot air is introduced at the die and the hot air breaks up the extruded polymer into small non-continuous fibres. The fibres are blown on to a drum where they form a web. The drum rotates continuously so that as the web is formed, the web is separated from the drum. The separated web is taken through spreader rolls and is then wound onto a take-up roll. The size of the fibres and the density of the web can be controlled by the type of polyolefin used, the temperature to which polyolefin granules are melted in the extruder or meltbox, the temperature and pressure of the air at the die, the cool air quenching the fibres between the die and the drum, and the distance of the drum from the die. The filtration and the pressure drop can be controlled by machine settings during the production of the meltblown polyolefin fibres. Thus, for example, it is possible to compact the polyolefin fibres onto the drum and therefore produce polyolefin fibres which have a high efficiency and a high pressure drop when they are used for filtration purposes. It is also possible lightly to bond the meltblown polyolefin fibres as they hit the drum, in order to give a lower pressure drop and a lower efficiency filtration. The art of producing meltblown polyolefin fibres for use in filters is to achieve the best compromise from the available options, for any specific type of filter for which the meltblown polyolefin fibres are intended.
It is known to improve meltblown polyolefin fibres for filtration purposes by electrostatically charging the fibres. The electrostatically charging of the fibres makes it possible to have filters which have a low pressure drop and a high efficiency. However, electrostatically charged meltblown polyolefin fibres are affected by moisture. The electrostatic charge and therefore the performance of the filter become lost over a period of time in conditions above 70% humidity. Since filters are rarely re-tested once in use, any filter which falls below the minimum standard represents a hazard to the user.
It may generally be said that polyolefin fibres are safer to use from a health point of view than glass fibres, but the polyolefin fibres tend not to work as efficiently as the glass fibres. Fibres made from glass have a diameter of less than 5 microns in size, and they are thus fine enough to produce the required efficiency/pressure drop. Also, they are not affected by moisture. The polyolefin fibres do not lead to health problems as may the glass fibres, However known processes for producing polyolefin fibres are such that the polyolefin fibres are only able to be produced with diameters of not more than 5 microns in size because either the polyolefin becomes too brittle and breaks down, or else the fibres are so small that they do not have enough inertia to collect on the drum and form the web. If the fibres are so small, when they are blown towards the drum, they simply become airborne and do not collect on the drum. Thus difficulties have been encountered in producing polyolefin fibres of a diameter effectively less than an equivalent diameter to glass, that in less than 5 microns diameter. Therefore high performance mechanical filters using meltblown polyolefin fibres have generally not been available.
In an attempt to provide polyolefin fibres which do not suffer from the above problems, European Patent No. 0616831 discloses adding an electrostactically charged fluorochemical to a polyolefin fibre having a melt flow index of up to 400. The fluorocarbon additive remains within the fibres as they are produced by the meltblown process. For the fluorochemical to act to increase the filtration efficiency of the fibres, it is necessary for the fluorochemical to be on the surface of the fibres. In order to achieve this in the European patent, the fibres have to be provided with a second process step of annealing. More specifically, the formed web of meltblown polyolefin fibres is annealed for ten minutes at 140.degree. C. in order to get the fluorochemical to migrate to the surface of the fibre, where it appears as a bloom on the surface of the fibres. The disadvantage of this separate annealing step is that it is cumbersome and it adds to the expense of the production of the meltblown polyolefin fibres.